The present invention relates to a discharge lamp and a lamp unit. In particular, the present invention relates to a discharge lamp and a lamp unit used as the light source of an image projection apparatus such as a liquid crystal projector or a digital micromirror device (DMD) projector.
In recent years, an image projection apparatus such as a liquid crystal projector or a projector using a DMD has been widely used as a system for realizing large-scale screen images. A high-pressure discharge lamp having a high intensity has been commonly and widely used in such an image projection apparatus. For the light source used in the image projection apparatus, light is required to be concentrated on an imaging device included in the optical system of the projector, so that in addition to high intensity, it is also necessary to achieve a light source close to a point light source. Therefore, a short arc ultra high pressure mercury lamp that is closer to a point light and has a high intensity has been noted widely as a promising light source.
Referring to FIG. 4, a conventional short arc ultra high pressure mercury lamp 1000 will be described. FIG. 4 is a schematic view of an ultra high pressure mercury lamp 1000. The lamp 1000 includes a substantially spherical luminous bulb 110 made of quartz glass, and a pair of sealing portions (seal portions) 120 and 120xe2x80x2 also made of quartz glass and connected to the luminous bulb 110.
A discharge space 115 is inside the luminous bulb 110. A mercury (in an amount of, for example, 150 to 250 mg/cm3) as a luminous material, a rare gas (e.g., argon with several tens kPa) and a small amount of halogen are enclosed in the discharge space 115. A pair of tungsten electrodes (W electrode) 112 and 112xe2x80x2 are opposed with a certain electrode distance D (e.g., about 1.5 mm) in the discharge space 115. Each of the W electrodes 112 and 112xe2x80x2 includes an electrode axis (W rod) 116 and a coil 114 wound around the head of the electrode axis 116. The coil 114 has a function to reduce the temperature at the head of the electrode.
The electrode axis 116 of the W electrode 112 is welded to a molybdenum foil (Mo foil) 124 in the sealing portion 120, and the W electrode 112 and the Mo foil 124 are electrically connected by a welded portion where the electrode axis 116 and the Mo foil 124 are welded. The sealing portion 120 includes a glass portion 122 extending from the luminous bulb 110 and the Mo foil 124. The glass portion 122 and the Mo foil 124 are attached tightly so that the airtightness in the discharge space 115 in the luminous bulb 110 is maintained. In other words, the sealing portion 120 is sealed by attaching the Mo foil 124 and the glass portion 122 tightly for foil-sealing. The sealing portions 120 have a substantially circular cross section, and the rectangular Mo foil 124 is disposed in the center of the inside of the sealing portion 120.
The Mo foil 124 of the sealing portion 120 includes an external lead (Mo rod) 130 made of molybdenum on the side opposite to the side on which the welded portion is positioned. The Mo foil 124 and the external lead 130 are welded to each other so that the Mo foil 124 and the external lead 130 are electrically connected at a welded portion 132. The configurations of the W electrode 112xe2x80x2 and sealing portion 120xe2x80x2 are the same as those of the W electrode 112 and sealing 120, so that description thereof will be omitted.
Next, the operational principle of the lamp 1000 will be described. When a start-up voltage is applied to the W electrodes 112 and 112xe2x80x2 via the external leads 130 and the Mo foils 124, discharge of argon (Ar) occurs. Then, this discharge raises the temperature in the discharge space 115 of the luminous bulb 110, and thus the mercury is heated and evaporated. Thereafter, mercury atoms are excited and become luminous in the arc center between the W electrodes 112 and 112xe2x80x2. The higher the mercury vapor pressure of the lamp 1000 is, the higher the emission efficiency is, so that a lamp having a higher mercury vapor pressure is more suitable as a light source for an image projection apparatus. However, in view of the physical strength against pressure of the luminous bulb 110, the lamp 1000 is used at a mercury vapor pressure of 15 to 25 MPa.
The conventional lamp 1000 is produced in the manner as shown in FIGS. 5A to 5C. FIGS. 5A to 5C are cross-sectional views showing a production process sequence of a method for producing the lamp 1000.
First, a glass pipe 150 for a discharge lamp having a luminous bulb portion 110 that will be formed into the luminous bulb of the lamp 1000 and a side tube portion (sealing portion) 122 that will be formed into the sealing portion of the lamp 1000, and an electrode assembly 140 in which the electrode 112 is joined to one end of the metal foil (Mo foil) 124 and the external lead 130 is joined to the other end are prepared. Then, as shown in FIG. 5A, the electrode assembly 140 is inserted in the glass pipe 150 for a discharge lamp (electrode assembly insertion process).
Next, as shown in FIG. 5B, when the pressure in the glass pipe 150 is reduced (e.g., less than 1 atmospheric pressure), and the glass tube 122 of the glass pipe 150 is heated and softened with, for example, a burner 54, so that the side tube portion 122 and the Mo foil 124 are attached tightly, thereby forming the sealing portion 120 (sealing portion formation process).
The same processes as those shown in FIGS. 5A and 5B are performed to the other side tube portion. More specifically, another electrode assembly 140 is inserted into a side tube portion that has not been formed into a sealing portion yet. At this time, the electrode assembly 140 is inserted while being aligned with the electrode 112 of the already-sealed electrode assembly 140 in such a manner that the pair of electrodes are on the same axis as much as possible and a predetermined electrode distance D is achieved. Thereafter, the sealing portion formation process is performed.
In this manner, when the sequence of the electrode assembly insertion process and the sealing portion formation process is performed twice, the luminous bulb 110 in which the pair of electrodes 112 are arranged in the discharge space 115 sealed with the pair of sealing portions 120 can be formed, as shown in FIG. 5C. Thus, the lamp 1000 can be produced. The luminous material enclosed in the discharge space 115 can be introduced into the luminous bulb 110 after one sealing portion 120 is formed and before the other sealing portion 120 is formed.
The electrode distance D of the lamp 1000 is a very important design matter that defines the arc length of the discharge lamp. When the electrode distance D of the lamp 1000 is short, a discharge lamp serving as a light source closer to a point light source and having higher intensity can be realized. However, the inventors of the present invention found that there are limitations of the conventional production method regarding further reduction of the electrode distance D. More specifically, the inventors of the present invention found limitations in the production process as follows. In the conventional production method, it is necessary to define the electrode distance D in the electrode assembly insertion process shown in FIG. 5A, so that the electrode distance D cannot be defined with a higher precision than that of the alignment in the electrode assembly insertion process.
Since the electrode assembly 140 has a configuration where the W rod 116 and the external lead 130 are joined to ends of a thin Mo foil 124 (e.g., a thickness of about 20 to 30 xcexcm), it is difficult to improve the alignment precision because of the small thickness of the Mo foil 124. Therefore, when the lamp 1000 is produced by the conventional production method, the short arc lamp 1000 that can be obtained has an electrode distance D of about 1.5 mm to 1.2 mm at best, and it is technically very difficult to realize a short arc lamp 1000 having a distance D between the electrodes shorter than that.
Therefore, with the foregoing in mind, it is a main object of the present invention to provide a method for producing a discharge lamp that can define the electrode distance between a pair of electrodes with high precision.
A method for producing a discharge lamp of the present invention includes the steps of: preparing a glass pipe for a discharge lamp having a luminous bulb portion and a side tube portion, and a single electrode assembly including an electrode structure portion that will be formed into a pair of electrodes of the discharge lamp; inserting the single electrode assembly into the glass pipe for a discharge lamp such that the electrode structure portion of the single electrode assembly is positioned in the luminous bulb portion of the glass pipe for a discharge lamp; forming a luminous bulb in which the electrode structure portion is arranged inside by attaching the side tube portion of the glass pipe for a discharge lamp to a part of the single electrode assembly; and forming a pair of electrodes in the luminous bulb by melting and cutting a part of the electrode structure portion selectively.
It is preferable that the electrode structure portion has a configuration in which the pair of electrodes of the discharge lamp are on the same axis.
In one embodiment of the present invention, the method for producing a discharge lamp further includes the step of filling a luminous material into the luminous bulb portion of the glass pipe for a discharge lamp.
In one embodiment of the present invention, the method for producing a discharge lamp further includes the step of filling halogen or halogen precursor into the luminous bulb portion, wherein after melting and cutting the part of the electrode structure portion, the step of cleaning the inside of the luminous bulb in which the pair of electrodes are formed is performed by the halogen or halogen derived from the halogen precursor.
In one embodiment of the present invention, the step of cleaning the inside of the luminous bulb includes the step of vacuum-baking the luminous bulb to cause halogen cycles with the halogen.
It is preferable that the single electrode assembly includes a single tungsten rod serving as the electrode structure portion and metal foils joined to both ends of the single tungsten rod.
It is preferable that coils are wound around both sides of a part of the single tungsten rod that is to be melted and cut selectively.
It is preferable that the step of forming the pair of electrodes is performed by irradiation of laser light from the outside of the luminous bulb.
It is preferable that the irradiation of the laser light is performed by rotating the luminous bulb portion relatively.
The step of forming the pair of electrodes may be performed by allowing current to flow through the single electrode assembly.
It is preferable that the step of forming the pair of electrodes is performed while cooling the luminous bulb.
It is preferable that the step of forming the pair of electrodes is performed while cooling the portions that will be formed into the base portions of the pair of electrodes when the electrode structure portion is formed into the pair of electrodes.
In one embodiment of the present invention, the step of attaching the side tube portion to a part of the single electrode assembly includes the step of preliminarily attaching the side tube portion to the part of the electrode assembly such that a gap is generated between the electrode structure portion and the side tube portion, and after the step of the preliminary attachment, the part of the electrode structure portion is melted and cut selectively.
It is preferable that the gap has a length that can prevent the electrode structure portion from being in contact with the side tube portion, even if the electrode structure portion is expanded by heat during melting and cutting.
In one embodiment of the present invention, the method for producing a discharge lamp further includes the step of melting and cutting the part of the electrode structure portion selectively and then adjusting an electrode distance between the pair of electrodes obtained by melting and cutting, after the step of the preliminary attachment.
In one embodiment of the present invention, the method for producing a discharge lamp further includes the step of attaching a part of each of the pair of electrodes to the side tube portion so as to fill the gap, after the part of the electrode structure portion is melted and cut selectively.
According to another aspect of the present invention, a discharge lamp includes a luminous bulb in which a luminous material is enclosed and a pair of electrodes are opposed to each other in the luminous bulb; and a pair of sealing portions for sealing a pair of metal foils electrically connected to the pair of electrodes, respectively. The discharge lamp is produced by a method including the steps of preparing a glass pipe for a discharge lamp having a luminous bulb portion and a side tube portion, and a single electrode assembly including an electrode structure portion that will be formed into a pair of electrodes of a discharge lamp; inserting the single electrode assembly into the glass pipe for a discharge lamp such that the electrode structure portion of the single electrode assembly is positioned in the luminous bulb portion of the glass pipe for a discharge lamp; forming a luminous bulb in which the electrode structure portion is arranged inside by attaching the side tube portion of the glass pipe for a discharge lamp to a part of the single electrode assembly; and forming a pair of electrodes in the luminous bulb by melting and cutting a part of the electrode structure portion selectively, wherein an electrode distance between the pair of electrodes is 1 mm or less.
In the present invention, a part of the electrode structure portion of the electrode assembly is melted and cut selectively to form a pair of electrodes in the luminous bulb. Therefore, the distance between the pair of electrodes can be defined with a higher precision than that in the prior art. As a result, a discharge lamp having a shorter electrode distance (e.g., 1 mm or less, preferably 0.8 mm or less) that could not be realized in the prior art can be produced.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.